This invention relates to an anti-lock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
U.S. application Ser. No. 789,576 filed on Oct. 21, 1985 and assigned to the assignee of this invention describes a wheel lock control system for preventing the wheels of a vehicle from locking up while being braked. In this system, the wheel brake pressure that results in the wheel slip being at the critical slip value and which produces the maximum braking force between the tire and the road surface is identified. When an incipient wheel lockup condition is detected, the brake pressure so identified is then applied to the wheel brake so as to substantially continuously establish the critical slip value between the wheel and the road surface resulting in the maximum possible braking effort.
The brake pressure producing the critical slip value and therefore the maximum braking force is identified in the above system by repeatedly calculating the braking force between the wheel and the road surface during braking based on an equation defining the motion of a free body consisting of the wheel, tire and the brake. This equation utilizes measured values and system constants that are based on, for example, brake lining coefficient of friction and area and wheel radius. The brake pressure corresponding in time to the peak calculated force is stored. When an incipient wheel lockup is detected indicating that the critical wheel slip value and therefore the peak braking force between the wheel and road surface has been exceeded, the stored brake pressure that produced the peak braking force is reestablished to establish a braking condition in which the wheel slip is substantially at the critical slip value for the existing road-tire interface condition.
As long as the relationship between the constants assumed in the equation for determining the braking force are correct, the wheel brake pressure establishing the maximum braking force between the tire and the road surface is accurately determined. However, the constants and their relationship establishing optimum braking for a new vehicle may not remain optimum over the life of the vehicle or for all braking conditions. For example, the brake lining coefficient of friction may vary with use and with operating conditions such as temperature and the tire characteristics may change with wear. These variations in the parameters upon which the constants were based may affect the determination of the peak braking force and therefore the brake pressure establishing the critical slip value.
It would therefore be desirable to adjust the constants as the parameters upon which they were based change so that the brake pressure establishing the critical slip value during wheel lock control braking is accurately determined over the life of the vehicle and for all braking conditions.